1. Field of the Invention
The present invention relates to a data format, a method of transferring data and a data transfer system.
2. Description of the Prior Art
“SDTI-CP: is described in the paper “Intra-Studio Distribution of Programme Bitstreams” by J. H Wilkinson and H. Nakano and published in/at IBC 99 Amsterdam 10-14 Sep. 1999.
The SDTI (Serial Data Transport Interface) is defined in SMPTE 305M. It is also reviewed in the paper “Interconnectivity in the DTV Era: The Emergence of SDTI” by A. Legault and J. Matey and published in/at IBC 99 Amsterdam 10-14 Sep. 1999.
Referring to FIG. 1, SDTI transmits packets in a signal structure comprising frames of television lines. Ancillary data (ANC) is carried in the horizontal blanking area (H-ANC) of lines and data is carried in a payload area of each line. The payload area is in the active line interval. SDTI allows packets to be routed wherever SDI connections are available and also allows packets from more than one source to be transmitted. SDI is the Serial Digital Interface defined by ITUR 656.
The SDTI-CP paper proposes that “Content Packages” (CP) are used to transfer primarily MPEG-2 encoded bitstreams using the SDTI. However other bitstreams can also be transferred.
A content package has a structure as shown in FIG. 2.
Each Content Package contains a video frame and associated audio and auxiliary data as shown in FIG. 1. It is constructed of up to four Items each comprising one or more Elements. The four Items are System, Picture, Audio and Auxiliary. There is always a System Item and at least one other Item. Picture and Audio Items are essentially stream Elements carrying the primary components of television. These two Items are often routed to specialist storage or processing equipment. Auxiliary data is used to carry data-centric content such as sub-titles, teletext and closed caption data and is frequently created, processed and stored on computer media. As digital delivery services increase, Auxiliary data types can be expected to grow in number, volume and complexity. Finally, the system Item provides services for the package as a whole through package metadata such as time stamps, metadata for Elements in the other three Items and finally, a downstream package control element.
The Picture, Audio and Auxiliary data Items can consist of up to 255 Elements which represents a potentially massive streaming capability. Each Content Package contains the associated contents of one frame period starting with a System Item and optionally containing Picture, Audio and Auxiliary Items. FIG. 2 shows the layered structure of a Content Package.
The metadata contained in the System Item has a link which associates any metadata Item uniquely with its associated element. In many cases, metadata is contained in the element (e.g. in the case of MPEG-2, metadata is contained in the various headers of the MPEG-2 bitstream). The external metadata link is provided to either replicate the metadata embedded in the Element or to provide additional metadata not contained in the element. Replicating the embedded metadata from an Element can provide quick access to key metadata without the need to re-parse the element bitstream.
Each content package CP must contain a System Item and the package is completed with any of the other Items. The System Item must appear first in order to provide a recognisable starting point and there can only be one Item of any type in any one package. A typical arrangement of System, Picture, Audio and Auxiliary data Items in a video field is shown in FIG. 1.
Each Item as described above is formatted as an SDTI ‘Variable Block’. The format of each SDTI variable block is shown in FIG. 3. The SDTI variable block has an ENDCODE and a SEPARATOR word as shown in FIG. 3. The SEPARATOR word is followed by a DATATYPE word which words together act as a START CODE.
Each block begins with the ‘Separator’ word and terminates with the “End Code” word. The “separator” word starts immediately following an SAV sequence. Thus, the start of a Content Package is defined by the detection of a System Item ‘Type’ code.
The SDTI-CP format as described in the paper is excellent for streaming video, audio and auxiliary data.
However, the present inventor has recognised that there is a need for a format which is compatible with computer files to allow data, primarily MPEG-2 encoded bitstreams but also other bitstreams, to be transferred, stored and manipulated as computer files. However, computer files have the limitation that they can only be accessed as a whole file.